In January 1968, a B52 plane carrying 4 nuclear weapon!: crashed on the sea ice similar to 12 km from the Thule Air Base, in northwest Greenland. The benthic marine environment in the 180-230 m deep Bylot Sound was then contaminated with similar to1.4 TBq Pu-239,Pu-240 (similar to0.5 kg). The site was revisited in August 1997, 29 years after the accident. Water and brown algae data indicate that plutonium is not transported from the contaminated sediments into the surface waters in significant quantities. Sediment core data only indicate minor translocation of plutonium from the accident to the area outside Bylot Sound. The present data support an earlier quantification of the sedimentation rate as 2-4 mm per year, i.e. 5-12 cm during the 29 years since the accident. Biological activity has mixed accident plutonium much deeper down, to 20-30 cm, and the 5-12 cm new sediment has been efficiently mixed into the contaminated layer. In addition to the classical bioturbation mixing the upper approximate to5 cm, the plutonium data indicate the existence of a deeper bioturbation gradually decreasing with depth. Transfer of plutonium to benthic biota is low leading to 1-2 orders of magnitude lower concentrations in biota than in sediments. Some biota groups show a somewhat higher uptake of americium than of plutonium. Sediment samples with weapons plutonium from the accident show a significant variation in Pu-240/Pu-239 atom ratios in the range 0.027-0.057. This supports the hypothesis that the Thule plutonium originates from at least two sources of different quality. The radioecological implication of the observed variations is that the use of plutonium isotope ratios in quantitatively determining the influence of different plutonium sources is a very complex affair requiring substantial data sets.
